Reducing water purification material, method for producing reducing water purification material, method for treating wastewater, and wastewater treatment apparatus

ABSTRACT

A reducing water purification material having a reducing iron-based precipitate selected from green rust, iron ferrite, reducing iron hydroxide, and a mixture thereof. A wastewater treatment process having steps of adding a reducing iron compound to wastewater, leading the wastewater to which the reducing iron compound is added to a reaction tank and forming a precipitate, separating the formed precipitate by a solid-liquid separation to obtain a sludge, and alkalinizing all or a portion of the separated sludge to form an alkaline sludge followed by returning to the reaction tank, wherein in the precipitation step, the wastewater to which the reducing iron compound is added and the alkaline sludge are mixed and are allowed to react in a non-oxidizing atmosphere under alkaline condition to form a reducing iron compound precipitate as the precipitate, thereby incorporating contaminants in the precipitate to remove the contaminants from the wastewater.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This is a divisional of U.S. application Ser. No. 10/598,494, filed Aug.31, 2006, which is a U.S. national phase application under 35 U.S.C.§371 of International Patent Application No. PCT/JP2005/008334 filedApr. 25, 2005, and claims the benefit of Japanese Application No.2004-130305, filed Apr. 26, 2004; Japanese Application No. 2004-251762,filed Aug. 31, 2004; Japanese Application No. 2004-263736, filed Sep.10, 2004; Japanese Application No. 2004-376581 filed Dec. 27, 2004 andJapanese Application No. 2004-376582, filed Dec. 27, 2004, all of whichare hereby incorporated by reference in their entireties. TheInternational Application was published in English on Nov. 3, 2005 asInternational Publication No. WO 2005/102942 A1 under PCT Article 21(2).

TECHNICAL FIELD

The present invention relates to a water purification material havingsuperior removal effects on heavy metals contained in wastewater andsuperior economy. More particularly, the present invention relates to awater purification material that can be used at normal temperatures,effectively removes heavy metals contained in wastewater and hassuperior economy, and to a production process thereof.

Moreover, the present invention relates to a purification treatmentsystem that efficiently removes contaminants from wastewater containingcontaminants, and has superior economy. More particularly, the presentinvention relates to a wastewater treatment process and treatmentapparatus that constitute a purification treatment system that uses areducing iron compound precipitate in the same manner as theaforementioned water purification material, having a simple process,superior practicality and superior economy for efficiently removingcontaminants contained in wastewater at normal temperatures.

Priority is claimed to Japanese Patent Application No. 2004-130305,filed on Apr. 26, 2004, Japanese Patent Application No. 2004-251762,filed on Aug. 31, 2004, Japanese Patent Application No. 2004-263736,filed on Sep. 10, 2004, Japanese Patent Application No. 2004-376581,filed on Dec. 27, 2004, and Japanese Patent Application No. 2004-376582,filed on Dec. 27, 2004, the contents of which are incorporated herein byreference.

BACKGROUND ART

A known example of a process of purifying wastewater containingcontaminants of the prior art consisted of removing heavy metal ionspresent in wastewater by reducing those metal ions by adding a reducingagent to the wastewater, and iron powder and so forth was used for thereducing agent.

For example, a process is described in Japanese Unexamined PatentApplication, First Publication No. H9-262592 in which a layer packedwith iron particles is formed in a column-shaped tank, and wastewater ispassed through this iron particle packed layer to remove heavy metals byadsorbing them onto the surface of the iron particles. However, inprocesses that use iron powder for the reducing agent, since theirreducing power decreases rapidly as a result of the surface reactionbeing impaired when heavy metals are adsorbed onto the surface of theiron particles, it is necessary to replace the iron powder at shortintervals, thereby resulting in the problem of a large maintenanceburden. Moreover, post-treatment is required under acidic conditions inparticular due to the generation of hydrogen gas and divalent iron. Inaddition, the packed layer becomes extraordinarily heavy due to the useof a large amount of iron powder, thereby placing a large burden on theapparatus structure as well.

In addition, selenium present in wastewater is subjected to strictdischarge standards as an environmental contaminant. Normally, seleniumis present in wastewater in the form of selenite ions (SeO₃ ²⁻)(tetravalent selenium) and selenate ions (SeO₄ ²⁻) (hexavalentselenium). Examples of known processes for removing this seleniuminclude: (i) a process in which a trivalent iron compound such as ferrichydroxide is added to co-precipitate the selenium by adsorbing it to aprecipitate by taking advantage of its aggregating action, (ii) aprocess in which barium or lead and so forth is added to form arefractory selenate precipitate, (iii) a process in which selenium isremoved by adsorbing using an ion exchange resin, and (iv) biologicaltreatment processes.

However, since co-prepitation by barium or lead is susceptible to theeffects of other ions present, it is necessary to increase the amountadded, and a burden is placed on post-treatment since barium and leadare also heavy metals. In addition, processes using an ion exchangeresin have the problem of their removal effects decreasing dramaticallyin the presence of sulfate ions and so forth. Moreover, biologicaltreatment processes have a long treatment time.

On the other hand, processes using trivalent iron compounds have hardlyany effects on hexavalent selenium. Therefore, a process has beenproposed that uses ferrous salt (divalent iron). This process promotesthe precipitation of selenium by reducing hexavalent selenium totetravalent selenium using the reducing power of ferrous iron.

For example, Japanese Unexamined Patent Application, First PublicationNo. H08-267076 describes a treatment process in which divalent iron ionsare added to selenium-containing wastewater followed by the addition ofan alkaline compound in an environment isolated from air while heatingand maintaining the liquid temperature to 30° C. or higher to form aselenium precipitate.

Japanese Unexamined Patent Application, First Publication No.2002-326090 describes a treatment process comprising a first step, inwhich hydroxides of heavy metals are precipitated by adding an alkalinecompound to selenium-containing wastewater, a second step, in which aninert gas is introduced into this treatment liquid to remove dissolvedoxygen followed by adding ferrous salt in the alkaline range to reduceand precipitate the selenium, and a third step, in which air is blowninto this treatment liquid to precipitate heavy metals remaining in theliquid by incorporating in an iron-containing precipitate.

Japanese Unexamined Patent Application, First Publication No. 2001-9467describes a treatment process which, on the one hand, forms aselenium-containing precipitate by adding ferrous hydroxide toselenium-containing wastewater and then adding an alkaline compound,while on the other hand, enhances treatment efficiency by circulating aportion of this sludge to a reaction tank following addition of analkaline compound.

However, it is difficult to lower the selenium concentration inwastewater to 0.01 mg/L or lower with the aforementioned treatmentprocesses of the prior art. In addition, in processes simply involvingthe addition of ferrous hydroxide, the treatment process is complicateddue to the need to preliminarily remove dissolved oxygen in thewastewater since oxygen in the wastewater competes for reaction withferrous ions with the selenium. Moreover, since precipitates of ferroushydroxide have a high moisture content and a large apparent density,they place a large burden on slurry treatment if used in that form.

Furthermore, although processes are known in which a portion of theformed precipitate is circulated to a reaction tank, since theconsolidating effects of precipitation are still low if the formedprecipitate is merely circulated, a burden is placed on post-treatment.Moreover, since many treatment processes of the prior art use ironferrite by heat-treating ferrous hydroxide, in addition to the treatmentprocess becoming complex, there is also the problem of increased heatingcosts.

In addition, a treatment process for removing heavy metals fromwastewater in which ferrous iron ions and so forth are added towastewater containing heavy metals, iron ferrite or pseudo-iron ferriteis formed by adjusting the pH to 5 or higher, and the formed ferritesludge is then separated into solid and liquid together with circulatingthe sludge by returning a portion to a reaction tank (JapaneseUnexamined Patent Application, First Publication No. 2001-321781).

This process focuses on the fact that ferrite sludge (FeO·Fe₂O₃)contains ferrous iron and ferric iron, and forms a precipitate byutilizing the fact that the presence of both ferrous iron and ferriciron more easily forms a ferrite sludge than ferrous iron alone.However, since the ferrite sludge of this treatment process has lowreducing power, there are limitations on its heavy metal removal effectseven if returned to a reaction tank.

On the other hand, in a wastewater treatment process in which a sludgeis precipitated by adding an alkaline compound to wastewater containingheavy metals followed by separation of this sludge, the alkalinecompound is not added directly to the heavy metal wastewater, but ratheris only added to a portion of the separated sludge, after which thisalkaline sludge is returned to a reaction tank (Japanese Examined PatentApplication, Second Publication No. S61-156, Japanese Unexamined PatentApplication, First Publication No. H05-57292 (Japanese Patent No.2910346)) However, it is difficult for the alkaline sludge alone tolower heavy metal levels to equal to or below environmental standardvalues.

In addition, magnetic separation means are known as means forefficiently separating heavy metal aggregates or heavy metalprecipitates when removing heavy metals contained in wastewater byprecipitation or aggregation.

Japanese Unexamined Patent Application, First Publication No.2000-117142 describes a means that aggregates heavy metal ions in wasteliquid, and a separation means that uses a magnetic filter to entrapparticles present in a waste liquid by forming a strong magnetic fieldwith a superconducting solenoid magnet.

Japanese Unexamined Patent Application, First Publication No.2001-321781 described a treatment process in which ferrite sludge isformed by adding ferrous iron ions to heavy metal wastewater followed byseparating with a thickener or magnetic separator and so forth.

Japanese Unexamined Patent Application, First Publication No.2001-259657 describes a treatment process in which magnetite particlesand so forth are added to form aggregates having increased magnetismfollowed by magnetic separation, that is used when aggregating and/orprecipitating phosphorous and heavy metals by theaggregation/precipitation and ferrite methods.

However, there are limitations on the magnetic separation effects ofmagnetic separation employed in the aforementioned treatment processesof the prior art since magnetic fields are applied statically in all ofthese processes. Since precipitates of heavy metals contained inwastewater are particularly diverse depending on the types andprecipitated states of the heavy metals, there is the problem of beingunable to obtain adequate separation effects simply by staticallyapplying a fixed magnetic field.

DISCLOSURE OF THE INVENTION

In order to solve the aforementioned problems of wastewater treatmentprocesses of the prior art using iron powder, a first object of thepresent invention is to provide a water purification material and itsproduction process in which reducing power is maintained for a longperiod, precipitates are consolidated, there is satisfactory separationof solids and liquids, and superior economic and treatment effects aredemonstrated enabling ferrite treatment at normal temperatures.

In order to solve the aforementioned problems by improving on treatmentprocesses based on ferrite processes of the prior art using ferroussalt, a second object of the present invention is to provide a treatmentprocess and treatment apparatus in which precipitates are consolidated,there is satisfactory separation of solids and liquids, superioreconomic and treatment effects are demonstrated enabling ferritetreatment at normal temperatures, and contaminants present in wastewaterare effectively removed by precipitating.

A third object of the present invention is to provide a treatmentapparatus that solves the aforementioned problems of the prior art byprecipitating heavy metals present in wastewater followed by theirfiltration and separation. In particular, a treatment apparatus isprovided in which precipitation of heavy metals or enhances solid-liquidseparation effects are promoted by applying a variable magnetic field toeither or both of a reaction tank in which heavy metals are precipitatedand a solid-liquid separation tank in which precipitates are separated.

A reducing water purification material of the present invention has areducing iron-based precipitate selected from green rust, iron ferrite,reducing iron hydroxide, and mixtures thereof.

In this reducing water purification material, a ratio of divalent ironto total iron (Fe²⁺/total Fe) in the aforementioned reducing iron-basedprecipitate may be 0.3 or more.

The reducing water purification material may have a slurry in which thereducing iron-based precipitate is dispersed in water, anoxidation-reduction potential of the slurry may be −500 mV to −800 mVversus Ag/AgCl electrode, and a pH of the slurry may be 7 to 11.

The reducing water purification material may be used for removing one ormore of selenium, copper, hexavalent chromium, molybdenum, boron,antimony, lead, arsenic, zinc, cadmium, nickel, manganese, fluorine,tin, phosphorous, cobalt, and organochlorine compounds oftrichloroethylene and dichloroethylene, which are contained in awastewater.

The reducing water purification material may be used by contacting withwastewater under neutral or alkaline condition.

The reducing water purification material may be used by contacting withwastewater in a non-oxidizing atmosphere.

According to the reducing water purification material of the presentinvention, heavy metals contained in wastewater are effectively removedfrom the wastewater by being incorporated in an iron-based precipitate.More specifically, concentration in wastewater of, for example,selenium, cadmium, chromium, lead, zinc, copper, or nickel can bereduced to less than 0.01 mg/L, while concentration in wastewater ofarsenic or antimony can be reduced to less than 0.001 mg/L. In addition,in the case of using this reducing water purification material, heatingis not required, and conversion of the precipitate to iron ferriteproceeds by incorporating wastewater heavy metals at normaltemperatures. Moreover, since consolidated, compact precipitates areformed due to the conversion to iron ferrite, the precipitates can bedewatered easily, thereby reducing the burden placed on post-treatmentby the precipitate resulting in superior economy and ease of handling.Here, since the precipitate is mainly including magnetite, it ismagnetic and can be treated by adsorbing the separated precipitate on amagnet.

A first aspect of the method for producing the reducing waterpurification material of the present invention has a step of adding analkaline compound to an aqueous solution of a ferrous salt to alkalinizeat a pH of 7 to 11, thereby forming an iron-based precipitate; a step ofseparating the iron-based precipitate by a solid-liquid separation andrecovering the iron-based precipitate, followed by further adding analkaline compound to adjust a pH to 11 to 13 to form a strong alkalineiron-based precipitate; a step of adding the strong alkaline precipitateto an aqueous solution of a ferrous salt, followed by adjusting a pH to7 to 11 and stirring to forma slurry; and a step of separating a formedprecipitate in the slurry by a solid-liquid separation to form aconcentrated precipitate, wherein a ratio of divalent iron to total iron(Fe²⁺/total Fe) in the slurry is made to be 0.3 or more, and anoxidation-reduction potential of the slurry is made to be −500 mV to−800 mV versus Ag/AgCl electrode, by repeating the separating, adding,and separating steps while adjusting contact surface areas with an airinterface.

A second aspect of the method for producing a reducing waterpurification material of the present invention has a step of aeratingwater with an inert gas to remove oxygen in the water; a step of addinga ferrous salt and a ferric salt to the water to form an aqueoussolution containing Fe²⁺ and Fe³⁺ at a molar ratio Fe²⁺/Fe³⁺ of 2; astep of adding an alkaline compound to the aqueous solution containingFe²⁺ and Fe³⁺ to adjust a molar ratio of hydroxide ions to total Fe to2, thereby forming a precipitate; a step of separating the formedprecipitate by a solid-liquid separation and recovering the precipitate,followed by further adding an alkaline compound to adjust a pH to 11 to13 to form a strong alkaline iron-based precipitate; a step of addingthe strong alkaline iron-based precipitate to an aqueous solution of aferrous salt, followed by adjusting a pH to 7 to 11 and stirring to forma slurry; and a step of separating a formed precipitate in the slurry bya solid-liquid separation to form a concentrated precipitate, wherein aratio of divalent iron to total iron Fe (Fe²⁺/total Fe) in the reducingiron-based precipitate is made to be 0.3 or more, and anoxidation-reduction potential of the slurry is made to be −500 mV to−800 mV versus Ag/AgCl electrode, by carrying out steps E to G in aninert gas atmosphere and repeating the separating, adding and separatingsteps while adjusting contact surface areas with an air interface.

The method for treating wastewater of the present invention is a methodfor treating wastewater which removes contaminants from wastewater byadding a reducing iron compound to wastewater containing contaminants toprecipitate the contaminants, followed by separating the precipitate bya solid-liquid separation to remove the contaminants from thewastewater, and which has a reducing iron compound addition step ofadding a reducing iron compound to wastewater; a precipitation step ofleading the waste water to which the reducing iron compound is added toa reaction tank, and forming a precipitate; a solid-liquid separationstep of separating the formed precipitate by a solid-liquid separationto obtain a sludge; and a sludge return step of alkalinizing all or aportion of the separated sludge to form an alkaline sludge, followed byreturning to the reaction tank, wherein in the precipitation step, thewastewater to which the reducing iron compound is added and the alkalinesludge are mixed and are allowed to react in a non-oxidizing atmosphereunder alkaline condition to form a reducing iron compound precipitate asthe precipitate, thereby incorporating contaminants in the precipitateto remove the contaminants from the wastewater.

In the method for treating waste water of the present invention, thereducing iron compound precipitate formed in the reaction tank may be amixture of green rust and iron ferrite, and the reducing iron compoundprecipitate may be formed so that a ratio of bivalent iron ions to totaliron ions an the reducing iron compound precipitate (Fe²⁺,/Fe(T)) is 0.4to 0.8.

A pH of the alkaline sludge returned to the reaction tank may beadjusted to 11 to 13, a pH in the reaction tank in which this alkalinesludge is mixed may be adjusted to 8.5 to 11, and the reducing ironcompound precipitate is formed in a non-oxidizing atmosphere.

A ferrous iron compound may be used for the reducing iron compound, andthe precipitate may be formed in a non-oxidizing atmosphere at a liquidtemperature of 10° C. to 30° C. while the reaction tank is sealed.

The method for treating wastewater of the present invention may alsohave a pretreatment step prior to the reducing iron compound additionstep. In the pretreatment step, an iron compound or an aluminum compoundis added to the wastewater prior to the iron compound precipitation stepto precipitate a hydroxide of iron or aluminum under alkaline condition,thereby at least any one of silicate ions, aluminum ions, and traces oforganic compounds is co-precipitated with the hydroxide, followed by theprecipitate being removed by filtration, and the reducing iron compoundaddition step, the precipitation step, the solid-liquid separation step,and the sludge return step may be carried out on the treated wastewaterfrom which the precipitate is removed.

The method for treating wastewater further may also have a step ofadding an iron compound or aluminum compound to wastewater containingcontaminants, and separating a formed precipitate by a solid-liquidseparation, prior to the reducing iron compound addition step, in thereducing iron compound addition step, a ferrous iron compound may beadded to the treated wastewater, in the precipitation step, in thereaction tank, the wastewater to which the reducing iron compound isadded and the alkaline sludge may be allowed to react at a pH of 8.5 to11 at a temperature of 10° C. to 30° C. for 30 minutes to 3 hours in thenon-oxidizing atmosphere isolated from air, in the sludge return step,an alkaline compound may be added to alkalinizing all or the portion ofthe separated sludge to adjust a pH of the sludge to 11 to 13, therebyforming the alkaline sludge, and concentrations of the contaminants inthe wastewater separated by a solid-liquid separation may be reduced byrepeating the precipitation step, the solid-liquid separation step, andthe sludge return step.

with respect to the sludge which is separated in the solid-liquidseparation step, a sludge which is not returned to the reaction tank maybe filtered and dewatered, and a filtrate may be discharged to anoutside, alternatively other wastewater may be passed through a residueto separate contaminants in the other wastewater by utilizing a reducingpower remaining in the residue.

According to the method for treating waste water of the presentinvention, a concentration of each heavy metal of selenium, cadmium,hexavalent chromium, lead, zinc, copper, nickel, arsenic, or antimony inthe wastewater can be reduced to 0.01 mg/L or less.

Here, the above-mentioned wastewater may be any water which includescontaminants in a wide range of water types, such as groundwater,industrial waste water, river water, or swamp water.

The wastewater treatment apparatus of the present invention has a tankin which a ferrous iron compound is added to wastewater; a sealedreaction tank having a non-oxidizing atmosphere for allowing thewastewater to which the ferrous iron compound is added to react; asolid-liquid separation means for subjecting a slurry which is extractedfrom the reaction tank to a solid-liquid separation to obtain a sludge;a tank in which an alkaline compound is added to the separated sludge toform an alkaline sludge; a line through which the alkaline sludge isreturned to the reaction tank; and lines which connect each of the tanksand the solid-liquid separation means, and a treatment system relatingto the method for treating wastewater of the present invention isformed.

This wastewater treatment apparatus may also have a tank in which aniron compound or aluminum compound is added to the wastewater to form aprecipitate, and a solid-liquid separation means to separate the formedprecipitate by a solid-liquid separation, prior to the tank in which thereducing iron compound is added to the wastewater.

This treatment apparatus may also have a means for applying a variablemagnetic field to either or both of the reaction tank and thesolid-liquid separation means, and a magnetic field is fluctuated toprecipitate heavy metals or to separate heavy metal precipitate.

The solid-liquid separation means may have a solid-liquid separationtank, and magnets may be arranged on either or both of a periphery ofthe reaction tank and a partition of the solid-liquid separation tank,and the magnets are rotated or vibrated to fluctuate the magnetic field.

A plurality of reaction tanks may be arranged in series, and a means maybe provided for applying a variable magnetic field to either or both ofone or more of the reaction tanks and the solid-liquid separation means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing an example of the treatment process ofthe present invention.

FIG. 2 is a graph showing the relationship between seleniumconcentration in wastewater and the number of treatment cycles inExamples 1 and 2.

FIG. 3 is a flow chart showing an example of the treatment steps of thepresent invention that include a pretreatment step.

FIG. 4 is a flow chart showing an example of the treatment steps of thepresent invention provided with a variable magnetic field means.

FIG. 5 is a flow chart showing an example of the treatment steps of thepresent invention provided with a variable magnetic field means andincluding a pretreatment step.

DETAILED DESCRIPTION OF THE INVENTION

The following provides an explanation of preferable embodiments of thepresent invention with reference to the drawings. The present inventionis not limited to the following embodiments, and for example,constituent features of these embodiments may be suitably combined.

The water purification material of the present invention is a reducingwater purification material having a reducing iron-based precipitateincluding green rust, iron ferrite, or a reducing iron hydroxide, or amixture of two or three thereof, and preferably is a reducing waterpurification material including a reducing iron-based precipitate inwhich the ratio of divalent iron to total iron (Fe²⁺/total Fe) is 0.3 ormore.

Green rust is a bluish-green substance in which hydroxides of ferrousiron and ferric iron form layers, has a structure in which anions areincorporated between the layers, and is represented by, for example, thefollowing formula (1).

[Fe^(II) _((6−x))Fe^(III) _(x)(OH)₁₂]^(x+)[A_(x/n)·yH₂O]^(x−)  (1)

(0.9<x <4.2, Fe²⁺/Total Fe=0.3 to 0.85)

(A: anion such as SO₄ ²⁻ or Cl⁻)

For example, this green rust is referred to as green rust (II) (GR(II))when A=SO₄ ²⁻ and x=2. Green rust is converted to iron ferrite by mildoxidation.

Although iron ferrite includes mainly magnetite (Fe^(II)OFe^(III) ₂O₃),a portion of the Fe(II) or Fe(III) maybe substituted with heavy metals.The reducing iron-based precipitate of the present invention can be thatin which heavy metal ions present in wastewater have been incorporatedinto green rust and then converted to iron ferrite while containingheavy metals in a portion thereof.

A reducing iron hydroxide is a precipitate which includes mainly an iron(II) hydroxide of divalent iron, and can be obtained by, for example,adding an alkaline compound to an aqueous ferrous salt solution in anon-oxidizing atmosphere to form a precipitate. This iron(II) hydroxidegradually decomposes to green rust due to mild oxidation under neutralor alkaline condition.

Iron-based precipitates are used in which the ratio of divalent iron tototal iron (Fe²⁺/total iron) in the precipitate is at least 0.3 or moreso as to have reducing power. In the case in which the ratio of divalentiron of the iron-based precipitate is lower than this, reducing power isweak making this unsuitable. Incidentally, as was previously mentioned,the aforementioned ratio of divalent iron (Fe²⁺/total iron) in greenrust or a mixture of green rust and iron ferrite is 0.3 to 0.85, andreducing power increases the higher the amount of divalent iron.Furthermore, since green rust is converted to iron ferrite by mildoxidation, the aforementioned ratio of divalent iron should normally be0.4 to 0.65, and preferably 0.5 to 0.6.

This water purification material has the aforementioned iron-basedprecipitate. The oxidation-reduction potential of a slurry in which thisprecipitate is dispersed in water is preferably −500 mV to −800 mVversus Ag/AgCl electrode, and more preferably −620 mV to −680 mV. Inaddition, the pH of the slurry is preferably 7 to 11, and morepreferably 9 to 10. In the case in which the oxidation-reductionpotential is higher than the aforementioned range, reduction capacitydecreases thereby preventing removal treatment of heavy metals. Inaddition, if the pH is lower than the aforementioned range, divalentiron ions elute causing poor water quality. On the other hand, if the pHis higher than the aforementioned range, reduction capacity decreases.

This water purification material can be produced in the manner describedbelow.

(A) An alkaline compound such as calcium hydroxide is added to anaqueous ferrous salt solution such as aqueous ferrous sulfate solutionfollowed by adjusting to an alkaline pH of 7 to 11 to form an iron-basedprecipitate.(B) This precipitate is recovered by liquid-solid separation after whichan alkaline compound such as calcium hydroxide is again added to adjustto a strongly alkaline pH of 11 to 13.(C) This strongly alkaline precipitate is added to an aqueous ferroussalt solution such as aqueous ferrous sulfate solution followed byadjusting to a pH of 7 to 11, preferably a pH of about 9.0 and stirringto form a slurry.(D) The formed precipitate is separated from the liquid to obtain aconcentrated precipitate.

The ratio of divalent iron to total iron (Fe²⁺/total iron) in the slurrycan be made to be 0.3 or more, preferably 0.4 to 0.65, and theoxidation-reduction potential can be made to be −500 mV to −800 mV,preferably −620 mV to −680 mV versus Ag/AgCl electrode by repeating theaforementioned steps (B) to (D) while adjusting the contact surface areawith the air interface. The resulting concentrated precipitate can beused as a water purification material of the present invention.

In addition, this water purification material can also be produced usingan aqueous solution containing divalent iron and trivalent iron in themanner described below.

(E) Water such as ion exchange water is aerated with an inert gas suchas 99.99% N₂ to remove the oxygen.(F) A ferrous iron salt such as FeSO₄·7H₂O and a ferric iron salt suchas Fe₂(SO₄)₃ are added to the aforementioned water such as ion exchangewater to prepare a solution containing Fe²⁺ and Fe³⁺ for which theirratio is Fe²⁺/Fe³⁺=2 (molar ratio).(G) An alkaline compound such as NaOH is then added to this aqueous ironsulfate solution (solution containing Fe²⁺ and Fe³⁺) and mixed. Theamount of an alkaline compound such as NaOH added is adjusted so thatthe ration of hydroxide ions to total Fe=2 (molar ratio). As a result,green rust (II) is formed as shown in the following reaction formula.

4Fe²⁺+2Fe³⁺+12OH⁻+SO₄ ²⁻→Fe₄Fe₂(OH)₁₂SO₄

Here, the aforementioned steps (E) to (G) are carried out in an inertgas atmosphere such as 99.99% N₂.

(H) This precipitate is then recovered by solid-liquid separation and analkaline compound such as calcium hydroxide is added again to adjust toa strongly alkaline pH of 11 to 13.(I) Aqueous ferrous sulfate solution is added to this strongly alkalineprecipitate followed by adjusting the pH to 7 to 11, preferably about9.0 and stirring to form a slurry.(J) The formed precipitate is then separated from the liquid to obtain aconcentrated precipitate.

The ratio of divalent iron to total iron in the slurry (Fe²⁺/total iron)can be made to be 0.3 or more, preferably 0.4 to 0.65, and theoxidation-reduction potential can be made to be −500 mV to −800 mV,preferably −620 mV to −680 mV, versus Ag/AgCl electrode by repeating theaforementioned steps (H) to (J) while adjusting the contact surface areawith the air interface. The resulting concentrated precipitate can beused as a water purification material of the present invention.

This water purification material is preferably used under neutral oralkaline condition of pH 7 to 11, and more preferably pH 9 to 10. In thecase of using this water purification material, there are no particularlimitations on the temperature at which it is used, and it can be usedeven at normal temperatures. In addition, this water purificationmaterial is adequately contacted with wastewater under neutral oralkaline condition. It can be contacted with wastewater eithercontinuously or in individual batches, and examples of methods that canbe used for the type of apparatus include a method in which wastewateris contacted with precipitate in a tank using a stirring tank, a methodin which the precipitate is filled into a packed column and contactedwith wastewater, and a method in which the precipitate is contacted withwastewater by allowing to flow using a fluid bed. A divalent iron-basedsalt such as ferrous sulfate or ferrous chloride is added as necessary.In addition, the reduction reaction can be further accelerated byadjusting to a non-oxidizing atmosphere.

As a result of contacting this water purification material andwastewater, heavy metals contained in the wastewater are precipitated bybeing incorporated in the aforementioned iron-based precipitate, andremoved from the wastewater. For example, heavy metal ions such ascadmium, lead, zinc, nickel, and manganese are incorporated in theprecipitate as a result of being substituted for the iron. In addition,oxyanions such as hexavalent selenium and hexavalent chromium areincorporated in the iron-based precipitate of the water purificationmaterial as a result of being reduced to tetravalent selenium or metalselenium, or trivalent chromium. Moreover, oxyanions other than seleniumand chromium, such as pentavalent arsenic and tetravalent arsenic areremoved from wastewater by being incorporated in the loose layeredstructure of green rust.

In this manner, as a result of contacting wastewater with theaforementioned water purification material, heavy metals in thewastewater are removed from the wastewater by being incorporated in theaforementioned iron-based precipitate resulting in purification of thewastewater. In the case in which the reducing power of the waterpurification material decreases due to the accumulation of heavy metalsfollowing repeated use of the water purification material, the waterpurification material should be taken out of the tank and replaced witha fresh water purification material having potent reducing power.

This water purification material is able to effectively remove heavymetals from wastewater as a result of heavy metals contained in thewastewater being incorporated in a precipitate. More specifically, theconcentration in wastewater of, for example, selenium, cadmium,chromium, lead, zinc, copper or nickel can be reduced to less than 0.01mg/L, while the concentration in wastewater of arsenic or antimony canbe reduced to less than 0.001 mg/L. In addition, in the case of usingthis reducing water purification material, heating is not required, andconversion of the precipitate to iron ferrite proceeds by incorporatingwastewater heavy metals at normal temperatures. Moreover, sinceconsolidated, compact precipitates are formed due to the conversion toiron ferrite, the precipitates can be dewatered easily thereby reducingthe burden placed on post-treatment by the precipitate resulting insuperior economy and ease of handling. Furthermore, since theprecipitate is mainly including magnetite, it is magnetic and can betreated by adsorbing the separated precipitate on a magnet.

Next, the following provides an explanation of a wastewater treatmentprocess and wastewater treatment apparatus of the present invention thatuses a reducing iron compound precipitate similar to the aforementionedwater purification material.

This wastewater treatment process is a treatment process that removescontaminants from. wastewater by adding a reducing iron compound towastewater containing contaminants to precipitate the contaminantsfollowed by separating the precipitate from the liquid, having a step inwhich a reducing iron compound is added to wastewater (reducing ironcompound addition step), a step in which the wastewater to which thereducing iron compound has been added is led to a reaction tank to forma precipitate (precipitation step), a step in which the formedprecipitate (sludge) is separated from the liquid (solid-liquidseparation step), and a step in which all or a portion of the separatedsludge is alkalized and then returned to the reaction tank (sludgereturn step) ; wherein, in the precipitation step, wastewater to whichthe reducing iron compound has been added is mixed with alkaline sludge,allowed to react in a non-oxidizing atmosphere under alkaline conditionto form a reducing iron compound precipitate, and contaminants areremoved from the wastewater by being incorporated in the precipitate.

A rough flow chart showing an example of this treatment process is shownin FIG. 1. A treatment apparatus pertaining to the flow chart shown inthis drawing is a treatment apparatus that treats wastewater containingcontaminants in the form of heavy metals, and is provided with a tank 10in which a reducing iron compound is added to the wastewater, a sealedreaction tank 30 containing a non-oxidizing atmosphere in whichwastewater to which the reducing iron compound is added is allowed toreact, a solid-liquid separation tank 40 as a means for solid-liquidseparation of the slurry extracted from the reaction tank 30, a tank 20in which an alkaline compound is added to the separated sludge, a linethat returns the alkaline sludge to the reaction tank 30, and lines thatconnect each of these tanks and the solid-liquid separation means.

This treatment process and the treatment apparatus (to be referred to asa treatment system) has superior removal effects on contaminantscontained in wastewater, such as one or more heavy metals selected fromselenium, cadmium, hexavalent chromium, lead, zinc, copper, nickel,arsenic, and antimony. Wastewater containing these heavy metals is ledto the addition tank 10 followed by addition of a reducing ironcompound. Examples of reducing iron compounds that can be used includeferrous iron compounds such as ferrous sulfate (FeSO₄) and ferrouschloride (FeCl₂) . The amount of ferrous iron compound added is suitablyan amount such that the concentration of Fe²⁺ ion is 400 to 600 mg/L.Wastewater to which has been added the reducing iron compound is led tothe reaction tank 30.

In the reaction tank 30, wastewater to which has been added the reducingiron compound is mixed with alkaline sludge returned from thesolid-liquid separation step. This alkaline sludge is adjusted to pH 11to 13 by adding an alkaline compound to all or a portion of theprecipitate (sludge) separated from a liquid in a later step. Examplesof alkaline substances that can be added include calcium hydroxide, rawlime, sodium hydroxide, and a mixture of two or more thereof. Thesealkaline substances are used in a powder state as the alkaline compound.Alternatively, the alkaline substances are dissolved in a solvent suchas water, thereafter used as the alkaline compound. The pH in thereaction tank 30 is adjusted to pH of 8.5 to 11, and preferably 9.0 to10, by mixing with the alkaline sludge. In the reaction tank 30, areducing iron compound precipitate is formed by mixing wastewater towhich has been added a reducing iron compound with the alkaline returnedsludge, and reacting in a non-oxidizing atmosphere. This iron compoundprecipitate is a mixture of green rust and iron ferrite, and is areducing precipitate. As was previously stated, green rust is abluish-green substance in which hydroxides of ferrous iron and ferriciron form layers, has a structure in which anions are incorporatedbetween the layers, and is represented by, for example, theaforementioned formula (1). In addition, iron ferrite is a compoundwhich includes mainly magnetite (Fe^(II)OFe^(III) ₂O₃). The treatmentsystem of the present invention uses a sealed reaction tank isolatedfrom the entrance of air, and allows the reaction to proceed in anon-oxidizing atmosphere and under alkaline condition of pH 8.5 to 11,and preferably pH 9.0 to 10, to form the aforementioned reducing ironcompound precipitate in the reaction tank 30. The liquid temperatureshould be about 10° C. to 30° C., and heating is not required. Thereaction time should be about 30 minutes to 3 hours.

Furthermore, even in the case of a treatment process that causes theformation of an iron compound precipitate by adding a ferrous ironcompound and an alkaline compound to wastewater containing heavy metals,if the reaction tank is not sealed as in the prior art, the reaction isnot carried out in a non-oxidizing atmosphere, or the degree ofalkalinity is stronger than the above range of pH, a precipitate havingthe aforementioned reducing power is not formed, thereby preventing theobtaining of effects similar to those of the present invention.

In this treatment system, a precipitate is preferably formed so that theratio of divalent iron ions to total iron ions (Fe²⁺/Fe(T)) of theaforementioned precipitate is 0.4 to 0.8, and the aforementioned ionratio is more preferably controlled to 0.55 to 0.65, so that theaforementioned iron compound precipitate including a mixture of greenrust and iron ferrite has reducing power. In the case in which thisratio is outside the aforementioned range, reduction of heavy metalsbecomes inadequate, or the settling properties of the precipitatedeteriorate, thereby making this undesirable. Heavy metals contained inwastewater are reduced and easily incorporated in the precipitate byforming the aforementioned reducing iron compound precipitate.

As a result of repeatedly returning the alkaline sludge to the reactiontank 30 and repeating the reaction with wastewater to which the reducingiron compound has been added, the initially deep bluish-greenprecipitate is gradually oxidized to green rust and then becomes blackdue to conversion to iron ferrite. Since reducing power is lost when themajority of the green rust is converted to iron ferrite, in thetreatment process of the present invention, the ratio of divalent ironions to total iron ions (Fe²⁺/Fe(T))of the aforementioned iron compoundprecipitate is controlled to within the aforementioned range to form aprecipitate having reducing power.

In this treatment system, by repeatedly separating the aforementionedreducing sludge (iron compound precipitate), alkalizing all or a portionof it and returning it to the reaction tank, reacting in a non-oxidizingatmosphere, and again precipitating the reducing sludge, since thesludge (precipitate) is converted to iron ferrite while maintaining itsreducing power, consolidation of the precipitate proceeds, and since theconcentration of the precipitate increases significantly, reduction ofheavy metals in the wastewater proceeds and removal effects areimproved. Here, the precipitate (sludge) including mainly iron hydroxidehas a high apparent density and places a large burden on wastewatertreatment. In addition, in the treatment process of the presentinvention, since the iron ferrite that forms a precipitate is mainlyincluding magnetite, it is magnetic and can be treated by adsorbing theseparated precipitate on a magnet.

The slurry that has been discharged from the reaction tank 30 is led toa solid-liquid separation means such as a thickener in which it isseparated by allowing the sludge to settle to the bottom of the tank.Heavy metals can be removed from the wastewater by solid-liquidseparation of this precipitate. In addition, as was previously stated,an alkaline compound is added to all or a portion of the sludge toadjust the pH to 11 to 13, then it is returned to the reaction tank 30and the precipitate formation reaction is repeated in the reaction tank30. The proportion of sludge that is returned (returned sludgecirculation ratio) should be determined so that the ratio of divalentions to total iron ions of the precipitate that forms in the reactiontank 30 (Fe²⁺/Fe(T)) is within the aforementioned range. Furthermore,the treatment process of the present invention can be carried out inbatches or continuously.

The following provides a specific example of this treatment system. Aferrous iron compound is added to and dissolved in wastewater having aninitial selenium concentration of 2 mg/L so as to adjust an Fe²⁺ ionconcentration to 400 to 600 mg/L. A precipitate slurry, of which the pHhas been adjusted to pH 11 to 13 by addition of an alkaline compound, ismixed into this wastewater to which the ferrous iron compound has beenadded, and allowed to react for 30 minutes to 3 hours at pH 9.0 to 9.3and at a temperature of 10° C. to 30° C. in a sealed reaction tankisolated from the entrance of air. By then repeatedly separating theresulting precipitate from a liquid, adding an alkaline compound to aportion of the precipitate and returning to the reaction tank, theselenium concentration in the wastewater can be reduced to 0.01 mg/L orless.

In the treatment system relating to the flow chart shown in FIG. 1, twoor more reaction tanks 30 should be arranged in series, the tanks shouldbe sealed from air by purging with nitrogen, and the aforementionedferrite conversion treatment should be carried out in a non-oxidizingatmosphere.

In the case in which silicate ions, aluminum ions, or traces of organiccompounds are additionally contained in the aforementioned wastewatercontaining contaminants, the aforementioned ferrite conversion may beaffected by these ions, thereby lowering contaminant removal effects.With respect to such wastewater, it is preferable to provide apretreatment step for removing silicate ions and so forth prior to thereducing iron compound addition step as shown in FIG. 3 wherein an ironcompound or aluminum compound is added to the wastewater to form aprecipitate, followed by filtering the precipitate.

In the aforementioned pretreatment step, by adding an iron compound towastewater containing contaminants and then adding an alkaline compoundto form an iron hydroxide under alkaline condition, at least one of thesilicate ions, aluminum ions, and traces of organic compounds areco-precipitated with the iron hydroxide precipitate, and thisprecipitate is removed from the wastewater by solid-liquid separation. Aferric iron compound such as ferric chloride is preferably for the ironcompound. An aluminum compound may be used instead of the iron compound.The aluminum compound is added to the wastewater followed by theaddition of an alkaline compound to precipitate aluminum hydroxide underalkaline condition. Since silicate ions and traces of organic compoundsare incorporated in this precipitate, they are removed from thewastewater by solid-liquid separation. When the aforementioned reducingiron compound addition step, precipitation step, solid-liquid separationstep, and sludge return step are carried out on treated wastewater fromwhich silicate ions, aluminum ions, or traces of organic compounds,which affect ferrite conversion, have been removed by this pretreatment,the aforementioned ferrite conversion is not inhibited, making itpossible to enhance the effects of removing heavy metals in thewastewater. The pretreatment step is preferably provided with a tank inwhich an iron compound or aluminum compound is added to the wastewater,and a liquid-separation means for the formed precipitate, prior to thetank in which a reducing iron compound is added to wastewater containingcontaminants.

In addition, as was previously stated, although all or a portion of thesludge separated in the solid-liquid separation means is returned tothere action tank after being alkalized, sludge that is not returned tothe reaction tank is dewatered by filtering with a filter press and soforth, after which the moisture is discharged outside the system. On theother hand, since the filtration residue still has residual reducingpower and satisfactory water permeability, wastewater from a separatesystem in which the degree of contamination is not high can be passedthrough this filtration residue as shown in FIG. 3 to degradecontaminants contained in this wastewater and remove them by utilizingthe reducing power remaining in the filtration residue.

According to this treatment process, the concentration of heavy metalsin wastewater can be reduced to 0.01 mg/L or less. Moreover, thistreatment process does not require heating, iron ferrite conversion canbe carried out at normal temperatures, and a consolidated, compactprecipitate is formed, the precipitate can be dewatered easily and heavymetal removal effects are high, thereby resulting in superior economyand ease of handling.

Next, an explanation is provided of a treatment apparatus capable ofpromoting precipitation or enhancing solid-liquid separation effects byapplying a variable magnetic field to either or both of theprecipitation reaction tank and the precipitate separation tank in theaforementioned treatment system.

An example of a treatment system having this variable magnetic fieldmeans is shown in FIGS. 4 and 5.

The treatment apparatus relating to the treatment steps of FIG. 4 isprovided with a tank led in which a reducing iron compound is added towastewater containing heavy metals, a sealed reaction tank 30 containinga non-oxidizing atmosphere in which heavy metals in the wastewater areprecipitated by reacting with the reducing iron compound, a tank 40 inwhich slurry extracted from the reaction tank 30 is subjected tosolid-liquid separation, a tank 20 in which an alkaline compound isadded to the separated sludge, a line for returning the alkaline sludgeto the reaction tank 30, and lines that connect each of the tanks andsolid-liquid separation means, wherein a means 50 that applies avariable magnetic field is provided at least in the reaction tank 30 orthe solid-liquid separation tank 40.

For example, a constitution may be employed in which a rotatable supportframe (not shown) is provided around the periphery of there action tank30 of or means 50 that applies a variable magnetic field, and magnets(not shown) are provided on the support frame so as to surround thereaction tank 30. As a result, a magnetic field can be formed thatincludes the inside of the reaction tank, and by rotating the supportframe, the magnetic field is made to rotate and fluctuate.Alternatively, a constitution may be employed in which a support frameis provided that oscillates up and down instead of the rotatable supportframe, and the magnetic field is made to oscillated up an down byoscillating up and down the support frame with the magnets attachedthereto. In addition, a constitution may be employed that is providedwith a plurality of electromagnets, wherein the magnetic field is madeto fluctuate electrically by switching the current applied to theelectromagnets. Since ferrite conversion proceeds within the reactiontank resulting in the formation of a magnetic precipitate, this ferriteconversion can be further promoted by applying a variable magneticfield.

In addition to the structure described above, a constitution may beemployed in which a support frame (not shown), for example, thatsurrounds the center of the inside of the tank, may be provided uprightfor magnetic field fluctuating means 50 of the solid-liquid separationtank 40, and electromagnets (not shown) are provided on the supportframe. By switching the current applied to the electromagnets, themagnetic field can be fluctuated by continuously forming a magneticfield. As a result of forming a magnetic field, aggregation of themagnetized precipitate is promoted, while settling of the aggregates ispromoted by canceling the magnetic field.

With the exception of the constitution relating to means 50 for applyinga variable magnetic field, the aforementioned treatment apparatus issimilar to the treatment apparatus relating to the treatment steps ofFIG. 1. Namely, wastewater containing heavy metals is led to theaddition tank 10 followed by theaddition of a reducing iron compound.Wastewater to which the reducing iron compound has been added is thenled to. the reaction tank 30.

In the reaction tank 30, alkaline sludge returned from a solid-liquidseparation step is mixed with the wastewater to which has been added thereducing iron compound. The pH of this alkaline sludge has been adjustedto pH 11 to 13 by the addition of an alkaline compound to all or aportion of the precipitate (sludge) that has been separated from theliquid in a subsequent step. The pH in the reaction tank 30 is adjustedto pH 8.5 to 11, and preferably 9.0 to 10, as a result of mixing in thisalkaline sludge. In the reaction tank 30, wastewater to which has beenadded the reducing iron compound is mixed with the alkaline returnsludge and allowed to react in a non-oxidizing atmosphere, resulting inthe formation of a reducing iron compound precipitate. This ironcompound precipitate is a reducing precipitate comprised of a mixture ofgreen rust and iron ferrite. A sealed reaction tank isolated from theentrance of air is used for the aforementioned reaction tank 30 in orderto form the aforementioned reducing iron compound precipitate. In thereaction tank 30, the reaction is carried out in a non-oxidizingatmosphere under alkaline condition of a pH of 8.5 to 11, and preferably9.0 to 10. The temperature is only required to be about 10° C. to 30°C., and heating is not required. The reaction time should be about 30minutes to 3 hours.

As a result of repeatedly returning the alkaline sludge to the reactiontank 30 and repeating the reaction with the wastewater to which has beenadded the reducing iron compound, the initially deep bluish-greenprecipitate is gradually oxidized to green rust and then becomes blackdue to conversion to iron ferrite. Since reducing power is lost when themajority of the green rust is converted to iron ferrite, the ratio ofdivalent iron ions to total iron ions (Fe²⁺/Fe(T))of the aforementionedprecipitate is preferably controlled to within the range of 0.55 to 0.65by causing the precipitate to form such that the aforementioned ratio is0.4 to 0.8.

By repeatedly separating the aforementioned reducing sludge (ironcompound precipitate), returning all or a portion thereof to thereaction tank after alkalizing, allowing to react in a non-oxidizingatmosphere and then again precipitating the reducing sludge,consolidation of the precipitate proceeds since the sludge (precipitate)is converted to iron ferrite while maintaining its reducing power, andthe concentration of the precipitate is increased considerably, therebyimproving the effect of removing heavy metals. In this manner, sinceconversion of the precipitate to ferrite proceeds within the reactiontank and the precipitate that forms is magnetic, this ferrite conversioncan be further promoted by applying a variable magnetic field.

Furthermore, a plurality of the aforementioned reaction tank 30 shouldbe provided in series, and the formed slurry should be transferred toeach tank in a stepwise manner to promote the ferrite conversionreaction. Variable magnetic field means 50 may be provided in any of thereaction tanks. Moreover, the upper portion of the reaction tank shouldbe of a form that is covered with a lid, and together with having asmall hole in the lid for insertion of a shaft member of a stirrer,should have a shape in which it inclines upward toward theaforementioned small hole. As a result of making the reaction tank tohave this form, the inside of the reaction tank that is continuous withthe outside air is limited to that which passes through the small hole,thereby maintaining the non-oxidizing atmosphere inside. In addition,since gas generated inside the tank is led to the small hole along theincline of the lid, it is able to escape to the outside through theminute gap around the aforementioned shaft member.

The slurry that has been discharged from the reaction tank 30 is led toa solid-liquid separation tank 40 such as a thickener in which it isseparated by allowing the sludge to settle to the bottom of the tank. Ameans 50 that applies a variable magnetic field is provided in thissolid-liquid separation tank 40, and by forming a variable magneticfield, aggregation of magnetized precipitate is promoted, whileprecipitation of the aggregate can be promoted by canceling the magneticfield.

The treatment apparatus relating to the treatment steps shown in FIG. 5is an example of a treatment apparatus provided with a constitutionrelating to a pretreatment step having a tank 60, in which a precipitateis formed by adding an iron compound or aluminum compound to wastewater,and a solid-liquid separation tank 70, which removes the precipitate,prior to the reducing iron compound addition tank 10. The other aspectsof this treatment apparatus are the same as the treatment apparatusrelating to the treatment steps shown in FIG. 4. Since silicate ions,aluminum ions and traces of organic compounds in the wastewater causeinhibition of the ferrite conversion in the reaction tank, by removingthese in advance in a pretreatment step, ferrite conversion proceedssmoothly and the effects of removing heavy metals can be enhanced.

EXAMPLE 1

Wastewater containing heavy metals was treated in the manner describedbelow using a batch system in accordance with the flow chart showing anexample of the treatment process of the present invention shown inFIG. 1. First, 2.0 L of wastewater containing contaminants (heavy metalconcentrations: 2 mg/L each) were led into the addition tank 10 followedby the addition of ferrous sulfate to an Fe(II) concentration of 600mg/L. On the other hand, the entire amount of separated precipitate wasreturned to the alkaline compound addition tank 20 followed by theaddition of 1.5 g of calcium hydroxide to adjust to a strongly alkalinepH of 12. This strongly alkaline precipitate was returned to thereaction tank, mixed with wastewater to which was added ferrous sulfateand allowed to react for 2 hours. Next, slurry extracted from thereaction tank was separated into solid and liquid by causing theprecipitate to settle by allowing to sett e undisturbed for 20 hours ina thickener. The entire amount of this precipitate was adjusted to astrongly alkaline pH as described above and returned to the reactiontank to repeat formation and separation of the precipitate 30 times. Thetreatment conditions and treatment results are shown in Table 1.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 3 Example 1Example 2 (Reducing iron compound addition step) ferrous iron compoundFeSO₄ FeCl₂ FeSO₄ FeSO₄ FeSO₄ added amount 600 mg/L 2000 mg/L as Fe(II)as Fe(II) (Alkaline compound Sludge not Calcium addition Step)containing hydroxide was calcium added to hydroxide was wastewaterreturned to before mixing reaction tank with return sludge Alkalinecompound Calcium Calcium Calcium Calcium Sludge not hydroxide hydroxidehydroxide hydroxide was containing Added amount 1.5 1.5 1.5 added toreaction calcium tank hydroxide was returned to reaction tank pH ofsludge slurry 12 12 12 pH of reaction pH of reaction tank: 9.0 to 9.3tank: 9.0 to 9.3 (Precipitation Step) Reaction time 120 min. 120 min. 60min. 60 min. 60 min. Temperature 20° C. 20° C. 20° C. 20° C. 20° C.Atmosphere isolation isolation isolation presence of air presence of airfrom oxygen from oxygen from oxygen pH 9.0 to 9.3 9.0 to 9.3 9.0 to 9.39.0 to 9.3 9.0 to 9.3 (solid-liquid Separation Step) Standing time 20hours 20 hours 20 hours 20 hours 20 hours amount of returned entireentire entire entire entire sludge amount amount amount amount amountFerrite conversion of A A A B B precipitate Consolidation of A A A C Cprecipitate Cocentrations of heavy metals in wastewater Se <0.01 <0.01<0.01 0.02 0.03 Cd <0.01 <0.01 <0.01 0.01 0.02 Cr(VI) <0.01 <0.01 <0.010.04 0.06 Pb <0.01 <0.01 <0.01 0.01 0.02 Zn <0.01 <0.01 <0.01 0.02 0.03Cu <0.01 <0.01 <0.01 0.04 0.06 As <0.01 <0.01 <0.01 0.02 0.03 Sb <0.01<0.01 <0.01 0.02 0.03 Ni <0.01 <0.01 <0.01 0.02 0.02 Note: In the table,FeSO₄ indicates FeSO₄•7H₂O, A indicates satisfactory results, Bindicates unsatisfactory results to some degree, and C indicates poorresults. The concentrations of heavy metals are shown by mg/L.

EXAMPLES 2 and 3 and COMPARATIVE EXAMPLES 1, and 2

Wastewater containing heavy metals was treated in the same manner asExample 1 with the exception of using the treatment conditions shown inTable 1. Those results are shown in Table 1. In addition, the seleniumconcentrations in the wastewater corresponding to the number oftreatment cycles in Example 1 and Example 2 are shown in the graph ofFIG. 2.

As shown in the results of Table 1, as a result of repeating thewastewater treatment according to the present treatment process, theprecipitate was converted to ferrite at normal temperatures, a highlyconsolidated precipitate was formed, and the heavy metal concentrationsin the wastewater were able to be reduced to the environmental standardof 0.0: mg/L or less.

EXAMPLE 4

Aqueous ferric chloride solution was added to a concentration of 1.0ml/L to 2 L of simulated wastewater containing 100 ppm each of silicateions and aluminum ions as well as 2 ppm of selenium. The pH of thewastewater was then adjusted to 8 to 8.5 by addition of an alkalinecompound to form a precipitate. After separating this precipitate byfiltration, the filtrate was treated in the same manner as Example 1.

On the other hand, results obtained by carrying out treatment in thesame manner as Example 1 without carrying out this pretreatment areshown in Table 2 for comparison.

As shown in Table 2, in wastewater in which the concentrations ofaluminum ions and silicate ions were reduced to 1 ppm and 15 ppm,respectively, by carrying out pretreatment, the concentration ofselenium was reduced to less than 0.01 ppm as a result of ferriteconversion treatment, and since ferrite conversion was allowed toproceed adequately, high removal effects were achieved.

On the other hand, the selenium concentration of wastewater notsubjected to pretreatment was 0.07 ppm after treatment, thusdemonstrating removal effects that were lower than in the case ofpretreatment.

TABLE 2 Raw wastewater After pretreatment Al³⁺ SiO₂ Se Al³⁺ SiO₂ Example4 100 ppm 100 ppm 2 ppm 1 ppm 15 ppm Comparative 100 ppm 100 ppm 2 ppmNo pretreatment sample After ferrite conversion treatment Ferriteconversion of Se precipitate Example 4 <0.01 ppm A Comparative   0.07ppm B sample Note: A indicates satisfactory results, and B indicatesunsatisfactory results to some degree.

EXAMPLE 5

Aqueous ferric chloride solution was added to a concentration of 1.0ml/L to 2 L of simulated wastewater containing 50 ppm of traces oforganic compounds (TOC) and 2 ppm of selenium followed by the additionof an alkaline compound to adjust the pH of the wastewater to 8 to 8.5and form a precipitate. The precipitate was separated by filtration andthe TOC concentration of the wastewater decreased to 20 ppm or less.This filtrate was treated in the same manner as Example 1.

On the other hand, results obtained by carrying out treatment in thesame manner as Example 1 without carrying out this pretreatment areshown in Table 3 for comparison.

As shown in Table 3, wastewater subjected to pretreatment demonstrated alow concentration of selenium, a volume ratio of concentrated sludge was20, the sludge was strongly magnetic, and ferrite conversion proceededadequately.

On the other hand, wastewater not subjected to pretreatment demonstrateda somewhat high selenium concentration, the volume ratio of concentratedsludge was 25, the sludge was weakly magnetic, and ferrite conversionwas inadequate.

Here, the volume ratio of concentrated sludge refers to a ratio of atotal volume of a slurry before settling to a volume of a sedimentationvolume of slurry after settling (volume ratio of concentratedsludge=(total volume of slurry before settling)/(sedimentation volume ofslurry after settling)).

TABLE 3 Raw wastewater After pretreatment TOC Se TOC Example 5 50 ppm 2ppm <20 ppm Comparative 50 ppm 2 ppm No pretreatment sample Afterferrite conversion treatment Volume ratio of Se Concentrated sludgeMagnetism Example 5 <0.01 20 Strong Comparative 0.08 25 Weak sampleNote: Volume ratio of concentrated sludge = (total volume of slurrybefore settling)/(sedimentation volume of slurry after settling)

EXAMPLE 6

Wastewater containing heavy metals was treated in the manner describedbelow using a batch system in accordance with the flow chart showing anexample of the treatment process of the present invention shown in FIG.3. First, 2.0 L of wastewater containing heavy metals (heavy metalconcentrations: 2 mg/L each) were led into the addition tank 10 followedby the addition of ferrous sulfate to an Fe(II) concentration of 600mg/L. On the other hand, the entire amount of separated precipitate fromthe liquid was returned to the alkaline compound addition tank 20followed by adding 1.5 g of calcium hydroxide to adjust to a stronglyalkaline pH of 12. This strongly alkaline precipitate was returned tothe reaction tank, mixed with wastewater to which was added ferroussulfate and allowed to react for 2 hours.

Next, the slurry extracted from the reaction tank was allowed to settleundisturbed for 20 hours in a thickener, thereby the precipitate wasseparated by sedimentation from a liquid. The entire amount of thisprecipitate was adjusted to a strongly alkaline pH as described aboveand returned to the reaction tank to repeat formation and separation ofthe precipitate 60 times. The resulting excess precipitate was filteredwith a filter press to obtain 790 g (wet weight) of a filtrationresidue.

When other wastewater containing heavy metal apart from theaforementioned wastewater containing heavy metal was adjusted to pH of9, and 2.0 L thereof was passed through this filtration residue, theconcentrations of the heavy metals in the wastewater all decreased to1/10 or less of their original concentrations as shown in Table 4. Theheavy metal concentrations before the wastewater was passed through thefiltration residue (before treatment) and after passing through thefiltration residue (after treatment) are shown in Table 4.

TABLE 4 (mg/L) Cd Cr Pb Cu Sb Zn Before 1 1 1 1 1 1 treatment After <0.1<0.1 <0.1 <0.1 <0.1 <0.1 treatment

EXAMPLE 7

Ferrous sulfate was added to 2 L of water to an Fe(II) concentration of600 mg/L to prepare a starting liquid. Calcium hydroxide was added tothis to adjust the pH to 9.0 and form a precipitate. This precipitatewas recovered by solid-liquid separation after which calcium hydroxidewas again added to adjust to a strongly alkaline pH of :2.

This strongly alkaline precipitate was added to an aqueous ferroussulfate solution containing 600 mg/L as Fe(II), followed by adjustingthe pH to 9.0 and stirring to prepare a slurry. At this time, thecontract surface area with. an air interface in a stirring device wasadjusted so that a ratio of divalent iron to total iron (Fe²⁺/Fe(T)) inthe slurry was 0.4 to 0.65, and an oxidation reduction potential was−620 mV to −680 mV versus Ag/AgCl electrode.

A concentrated precipitate was obtained by solid-liquid separation ofthe formed precipitate. The procedure in which this precipitate wasadjusted to a strong alkaline pH of about 12 followed by being added tothe aforementioned aqueous ferrous sulfate solution to obtain aconcentrated precipitate was repeated 25 times to obtain 0.38 L of aconcentrated precipitate slurry having a solid-liquid concentration of140 g/L.

2.0 L of simulated wastewater containing the metal ions shown in Table 1were contacted with this concentrated precipitate, a pH was adjusted toabout 9, and stirred for 2 hours followed by solid-liquid separation andmeasurement of the metal ion concentrations in the liquid. Those resultsare shown in Table 5.

As shown in Table 5, the concentrations of heavy metal ions in thewastewater treated with the water purification material of the presentinvention were reduced considerably. More specifically, theconcentrations in the wastewater of selenium, cadmium, chromium, lead,zinc, copper and nickel were all reduced to less than 0.01 mg/L, whilethe concentrations of arsenic and antimony in the wastewater werereduced to less than 0.001 mg/L. In addition, the concentrations ofmolybdenum, boron, manganese and fluorine were also reducedconsiderably.

TABLE 5 Treated Treated Raw water water Raw water water Element (mg/L)(mg/L) Element (mg/L) (mg/L) Se 2 <0.001 As 1 <0.001 Cu 1 <0.01 Zn 1<0.01 Cr 1 <0.01 Cd 1 <0.01 Mo 1 0.06 Ni 1 <0.01 B 2 0.88 Mn 1 0.03 Sb 1<0.001 F 10 5.2 Pb 1 <0.01

EXAMPLE 8

A precipitate was produced by carrying out the following procedure in aninert atmosphere. Ferrous sulfate and ferric sulfate were added to 2 Lof water aerated with inert gas to a concentration of 400 mg/L as Fe(II)and concentration of 200 mg/L as Fe(III) . Next, NaOH was added to thisto adjust the ratio of hydroxide ions/total Fe (molar ratio) to 2. Theprecipitate that formed as a result of this was recovered bysolid-liquid separation.

Using the precipitate produced in the aforementioned process as thestarting substance, the procedure of obtaining a concentratedprecipitate by adding the precipitate to an aqueous ferrous sulfatesolution was repeated. First, NaOH was added to the precipitate toadjust to a strong alkaline pH of about 12. This strongly alkalineprecipitate was added to aqueous ferrous sulfate solution containing 600mg/L as Fe(II), followed by adjusting the pH to 9.0 and stirring toprepare a slurry.

A concentrated precipitate was obtained by separating the precipitatethat formed from the liquid. The procedure of making this precipitatestrongly alkaline followed by obtaining a concentrated precipitate byaddition of the aforementioned aqueous ferrous sulfate solution wasrepeated 25 times while adjusting the contact surface area with the airinterface so that the ratio of divalent iron to total iron (Fe²⁺/totalFe) in the slurry was 0.4 to 0.65, and the oxidation-reduction potentialwas −620 mV to −680 mV versus Ag/AgCl electrode.

As a result, 0.38 L of concentrated precipitate slurry were obtainedhaving a solid-liquid concentration of 140 g/L. 2.0 L of simulatedwastewater containing the metal ions shown in Table 1 were contactedwith this concentrated precipitate and stirred for 2 hours followed bysolid-liquid separation and measurement of the metal ion concentrationsin the liquid. The results were similar to those of Example 7.

INDUSTRIAL APPLICABILITY

The water purification material of the present invention can be used atnormal temperatures, is able to effectively remove heavy metalscontained in wastewater, and offers superior economy. In addition, thewastewater treatment process and treatment apparatus of the presentinvention include a wastewater treatment system having a simple processand superior practicality as well as superior economy by being able toeffectively remove contaminants contained in wastewater at normaltemperatures with satisfactory efficiency.

1.-8. (canceled)
 9. A method for treating wastewater to removecontaminants from wastewater by adding a reducing iron compound towastewater containing contaminants to precipitate the contaminants,followed by separating the precipitate by a solid-liquid separation toremove the contaminants from the wastewater, the method comprising thesteps of: adding a reducing iron compound to wastewater; leading thewastewater to which the reducing iron compound is added to a reactiontank, and forming a precipitate; separating the formed precipitate by asolid-liquid separation to obtain a sludge; and alkalinizing all or aportion of the separated sludge to form an alkaline sludge, followed byreturning to the reaction tank, wherein in the precipitation step, thewastewater to which the reducing iron compound is added and the alkalinesludge are mixed and are allowed to react in a non-oxidizing atmosphereunder alkaline condition to form a reducing iron compound precipitate asthe precipitate, thereby incorporating contaminants in the precipitateto remove the contaminants from the wastewater.
 10. The method fortreating wastewater according to claim 9, wherein the reducing ironcompound precipitate formed in the reaction tank is a mixture of greenrust and iron ferrite, and the reducing iron compound precipitate isformed so that a ratio of bivalent iron ions to total iron ions in thereducing iron compound precipitate (Fe²⁺/Fe(T)) is 0.4 to 0.8.
 11. Themethod for treating wastewater according to claim 9, wherein a pH of thealkaline sludge returned to the reaction tank is adjusted to 11 to 13, apH in the reaction tank in which this alkaline sludge is mixed isadjusted to 8.5 to 11, and the reducing iron compound precipitate isformed in a non-oxidizing atmosphere.
 12. The method for treatingwastewater according to claim 9, wherein a ferrous iron compound is usedfor the reducing iron compound, and the precipitate is formed in anon-oxidizing atmosphere at a liquid temperature of 10° C. to 30° C.while the reaction tank is sealed.
 13. The method for treatingwastewater according to claim 9, wherein the method further comprises apretreatment step prior to adding the reducing iron compound step,adding an iron compound or an aluminum compound to the wastewater toprecipitate a hydroxide of iron or aluminum under alkaline condition,thereby at least any one of silicate ions, aluminum ions, and traces oforganic compounds is co-precipitated with the hydroxide, followed by theprecipitate being removed by filtration, and the reducing iron compoundaddition step, the precipitation step, the solid-liquid separation step,and the sludge return step are carried out on the treated wastewaterfrom which the precipitate is removed.
 14. The method for treatingwastewater according to claim 9, wherein the method for treatingwastewater further comprises a step of adding an iron compound oraluminum compound to wastewater containing contaminants, and separatinga formed precipitate by a solid-liquid separation, prior to the reducingiron compound addition step, wherein the reducing iron compound additionstep further comprises the step of, adding to a ferrous iron compound tothe treated wastewater, wherein the precipitation step further comprisesthe step of reacting , in the reaction tank, the wastewater to which thereducing iron compound is added and the alkaline sludge at a pH of 8.5to 11 at a temperature of 10° C. to 30° C. for 30 minutes to 3 hours inthe non-oxidizing atmosphere isolated from air, in the sludge returnstep, adding an alkaline compound to alkalinizing all or the portion ofthe separated sludge to adjust a pH of the sludge to 11 to 13, therebyforming the alkaline sludge, and reducing concentrations of thecontaminants in the wastewater separated by a solid-liquid separation byrepeating the precipitation step, the solid-liquid separation step, andthe sludge return step.
 15. The method for treating wastewater accordingto claim 9, wherein the contaminants contained in the wastewater are oneor more heavy metals selected from selenium, cadmium, hexavalentchromium, iron, zinc, copper, nickel, arsenic and antimony, and aconcentration of each heavy metal in the wastewater is reduced to 0.01mg/L or less.
 16. The method for treating wastewater according to claim9, wherein with respect to the sludge which is separated in thesolid-liquid separation step, a sludge which is not returned to thereaction tank is filtered and dewatered, and a filtrate is discharged toan outside, alternatively other wastewater is passed through a residueto separate contaminants in the other wastewater by utilizing a reducingpower remaining in the residue. 17.-21. (canceled)